Method for release of surface micromachined structures in an epitaxial reactor

ABSTRACT

A method for releasing from underlying substrate material micromachined structures or devices without application of chemically aggressive substances or excessive forces. The method starts with the step of providing a partially formed device, comprising a substrate layer, a sacrificial layer deposited on the substrate, and a function layer deposited on the sacrificial layer and possibly exposed portions of the substrate layer and then etched to define micromechanical structures or devices therein. The etching process exposes the sacrificial layer underlying the removed function layer material. Next there are the steps of cleaning residues from the surface of the device, and then directing high-temperature hydrogen gas over the exposed surfaces of the sacrificial layer to convert the silicon dioxide to a gas, which is carried away from the device by the hydrogen gas. The release process is complete when all of the silicon dioxide sacrificial layer material underlying the micromachined structures or devices is removed.

FIELD OF THE INVENTION

The present invention relates to the manufacturing of micromechanicaldevices, and relates in particular to a method for releasingmicromechanical structures from adjacent structures during manufacturein an epitaxial reactor.

BACKGROUND INFORMATION

One method of depositing structural layers during manufacture ofsurface-micromachined devices involves the use of an epitaxial reactor.Epitaxy is a process for producing layers of monocrystalline layers ofsilicon over a single crystal substrate, and for forming polycrystallinesilicon layers over other substrate materials, such as SiO₂ films onsilicon substrates. Epitaxial reactors may be operated with preciselycontrolled temperature and atmosphere environmental conditions to ensureuniform deposition and chemical composition of the layer(s) beingdeposited on the target substrate. In addition to precise control, useof an epitaxial reactor can permit build-up of layers on a substrate atsignificantly higher rates than typically found with LPCVD systems.

From U.S. Pat. No. 6,318,175, for example, one approach to creating amicromachined device such as a rotation sensor is to apply a sacrificialSiO₂ layer to a monocrystalline silicon substrate at a position whereone or more micromechanical deflection parts are to be formed. Windowsthrough the SiO₂ layer to the silicon substrate may then be formed byapplying a layer of photosensitive material, overlaying a mask patternover the photosensitive layer and exposing the masked surface to light,and then using a developer to selectively remove the light-exposedportion of the photosensitive material and HF to etch the SiO₂ layerdirectly underlying these exposed portions. Following creation of thedesired windows in the SiO₂ layer, an upper epitaxial layer of siliconmay then be deposited on both the SiO₂ layer and the contact openings.The upper epitaxial layer grows in polycrystalline form on the SiO₂layer, and in monocrystalline form on the contact window openings toprovide a direct connection to the silicon substrate. The structuralelements of the micromechanical device may be defined on the uppersilicon layer using, for example, an anisotropic plasma etchingtechnique. The etching may be performed through the polycrystallineportion of the epitaxial layer to the SiO₂ layer to form trenches aroundthe structural limitations of the micromechanical parts. Finally, theSiO₂ layer may be removed from beneath the micromechanical parts in theupper silicon layer during an etching process to complete the formationof the micromechanical device.

The final step of releasing the micromachined structures formed in theupper silicon layer from the underlying sacrificial silicon dioxidelayer may be problematic given the following: the geometry of themicromachined structures; the difficulty in ensuring complete etchantpenetration through the sacrificial layer beneath the structures; andproblems with device deformation and adhesion during the dry process.Release has been accomplished by etching using an HF vapor, as discussedin German Published Patent Application No. 19704454 and U.S. Pat. No.5,683,591, or by application of liquid HF in combination withsupercritical carbon dioxide (CO₂), to selectively release and evacuatethe sacrificial SiO₂ from underneath the micromachined structural parts.

These processes, however, may have associated disadvantages. Thechemically aggressive nature of HF may preclude its use in releasingmicromachined devices created on substrates cohabited by integratedcircuit portions. There may be potential damage due to liquid etchantsimpinging on delicate micromachined structures. There may be problemscreated by incomplete elimination of liquid etchants. There may beincreased process complexity and expense associated with process stepsrequiring removal and/or reinsertion of the devices from the epitaxialreactor. There may be a need to maintain the supply and environmentalcontrol of materials in special states in an epitaxial reactorenvironment.

Therefore there is a need for a less-complex, more cost-effective methodfor releasing micromachined structures from their underlying substrates.

SUMMARY OF THE INVENTION

According to an exemplary embodiment of the present invention, a methodfor releasing a micromachined structure or device from its supportingsubstrate may begin with a partially formed micromachine device, whichmay comprise a substrate layer of, for example, monocrystalline silicon,a sacrificial oxide-bearing layer of, for example, deposited or grownSiO₂ on the substrate layer and etched to create a pattern of holes oropen areas through to the substrate layer, and a functional layer of,for example, epitaxially deposited silicon which may be etched to definemicromechanical structures or devices thereon and thereby expose theunderlying sacrificial layer.

Once the elements of the micromechanical structure or device have beendefined in the function layer, an exemplary embodiment of the presentinvention provides for in situ cleaning of the device within theepitaxial reactor with both hydrogen (H₂) to remove surface oxides, andwith hydrochloric acid (HCl) to remove silicon residues and surfaceimperfections resulting from the trench etching process. Following thecleaning step, the structures may be released by exposure of the deviceto high temperature H₂, which bonds to the sacrificial layer'soxide-bearing material. The resulting gas may be flushed from the deviceby the H₂ flow. Exposure to the H₂ flow may be continued until all thesacrificial layer material beneath the elements of the micromechanicalstructure or device is evacuated.

The release of the micromechanical elements from their underlyingsacrificial layer supports in the foregoing manner may provide thefollowing advantages: avoiding the need to apply high forces on delicateelements; preventing adhering of the micromechanical element releasefrom the sacrificial layer (i.e., “sticking”); and reducing the need forhighly aggressive etching agents such as HF or liquid release agentswhose complete removal from the micromechanical structure or device maybe problematic. The use of H₂ in this manner may have the furtheradvantage of ready compatibility with an epitaxial environment andrelatively convenient handling of materials as compared to other releasesubstances such as acids, thereby simplifying process operations andenhancing epitaxial reactor production of the micromachined devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a through if show cross-sections and plan views of variousstages of production of an exemplary micromachined device.

FIGS. 2 a through 2 c illustrate the removal of sacrificial material torelease a micromechanical element in accordance with an exemplaryembodiment of the present invention.

FIG. 3 is a flowchart illustrating steps for releasing a micromechanicalelement in accordance with an exemplary embodiment of the presentinvention.

FIG. 4 shows a cross-section of an exemplary device having multiplelayers of a silicon bearing compound.

DETAILED DESCRIPTION

According to an exemplary embodiment of the present invention, a methodfor releasing a micromachined structure or device from its supportingsubstrate begins with a partially formed device, which may be formed asfollows. As shown in the cross-section view in FIG. 1 a, the partiallyformed device is based on a substrate layer 1 of silicon, upon which asacrificial layer 2 of SiO₂ is deposited or grown. FIG. 1 b shows across-section view of the substrate and sacrificial layer combination ofFIG. 1 a after a pattern of holes or open areas 3 have been formed insacrificial layer 2 using etching techniques, such as application of aphoto-sensitive material over the sacrificial layer, applying a maskwith the desired etching pattern over the photo-sensitive material,exposing the masked surface to light, and then applying an etchant toremove the exposed portions of the photo-sensitive material and thesacrificial SiO₂ underneath the exposed portions. FIG. 1 c shows a planview of the partially formed device of FIG. 1 b showing hole 3 definedby the etching process through sacrificial layer 2. The cross-sectionview in FIG. 1 b is taken through the line IB-IB of FIG. 1 c.

The partially formed device may then receive an epitaxially depositedfunction layer 4 of silicon, as shown in cross-section view 1 d. Theportions 5 of the fimction layer 4 formed on the SiO₂ have apolycrystalline structure, while the portions 6 of the function layer 4formed on the monocrystalline silicon substrate layer 1 have amonocrystalline structure. Function layer 4 is then etched to define themicromechanical structures or devices in function layer 4, with deep,narrow trenches 7 etched through the exposed portions of thephoto-sensitive material and the underlying polycrystalline silicon offunction layer 4, as shown in FIG. 1 e. FIG. 1 f is a plan view of thepartially formed device showing micromechanical element 8 defined byetched trenches 7. The cross-section views in FIG. 1 d and FIG. 1 e areboth taken through the line IE-IE, which corresponds to line IB-IB ofFIG. 1 c. The deflection beam portion 9 of micromechanical element 8 isshown in FIG. 1 e extending from the base portion 10 of micromechanicalelement 8. Base portion 10 is affixed to the silicon substrate 1, whiledeflection beam portion 9 may rest upon, and may therefore be restrainedby, an underlying column 11 of SiO₂ of sacrificial layer 2. This columnof sacrificial material must be removed to free beam 9 to deflect fromits rest position during operation of the micromechanical device.

Next, the surfaces of the partially formed device may be cleaned in situin the epitaxial reactor. In order to remove oxides on the surface ofthe device, H₂ at elevated temperature may be passed over the device,which may cause the oxide molecules to bond to the H₂ to form water andevaporate from the device surface. Following removal of any residualsurface oxides, gaseous HCl may be used to remove any remaining siliconresidues and surface imperfections from the surface of the device, suchas silicon residues remaining on the device surface during themicromechanical element-defining trench etching process.

Following the cleaning operation(s), the device may be exposed to H₂flowing at temperatures in the range of 800° C. to 1,400° C., as shownin FIG. 2 a. FIG. 2 a illustrates the exposure of the SiO₂ of thesacrificial layer in communication with the trenches 7 definingmicromechanical element 8 to the high temperature H₂ gas 12 flowing intoand out of the trenches 7. Upon reaching the exposed surfaces of theSiO₂ sacrificial layer around and underneath the micromechanicalelement, the gaseous H₂ may bond to oxygen in the oxide-bearing materialat the surface of the sacrificial layer, forming water (H₂O) and siliconmonoxide (SiO). The water and silicon monoxide are gaseous, andaccordingly may be immediately released from the exposed surface of thesacrificial layer into the flowing H₂ gas, which may sweep the water andsilicon monoxide out of the device. As illustrated in FIG. 2 b, therelease and removal of the gaseous water and gaseous silicon monoxidefrom the device trenches 7 may expose additional SiO₂ in the sacrificiallayer to the high temperature H₂ flow, causing additional SiO₂ to bereleased from the sacrificial layer. As illustrated in FIG. 2 c, thisprocess may continue until all the SiO₂ underlying the deflection beamportion 9 of micromechanical element 8 has been removed, and the beam isfreed.

The foregoing method may have the following advantages: permittingremoval of the sacrificial layer underlying the micromechanical elementswithout applying any significant impact force to the micromachinedelements; ensuring complete removal of the sacrificial layer materialunder the micromechanical elements from the device. This method mayavoid problems associated with incomplete drying of liquid agents fromwithin the semi-conductor device following sacrificial layer removal.

In another alternative exemplary embodiment, an SOI (Silicon onInsulator) wafer may be used to construct the device, where thesubstrate layer, the sacrificial layer, and the function layer maycollectively form the SOI wafer.

Increased production rates may result from higher etch rates and reducedhandling of the wafer. Sacrificial layer removal rates may be furtherenhanced by, for example, increasing the temperature of the H₂ used toconvert the SiO₂ to H₂O and SiO, or by introduction of small amounts ofgaseous germanium or gaseous silicon bearing compounds. Adding smallamounts of silicon carrier during the H₂ exposure may also be useful tomoderate the extent of silicon pitting at higher H₂ gas temperatures.The use of H₂ for SiO₂ removal in this exemplary embodiment may have theadvantages of ready compatibility with existing epitaxial equipment andhigh temperature environments, and relatively convenient handling ofmaterials. Accordingly, the method of the present embodiment may allowfor simplified process operations, further enhancing epitaxial reactorproduction of micromachined devices.

FIG. 3 is a flowchart showing a detailed implementation of the foregoingsteps for releasing a micromechanical element. The process method startsat step 100 with a device into which trenches have been etched to definean element of a micromechanical structure or device. In step 110, thesurface is chemically cleaned. This step may include removing residualmaterials from the trench etching process, removing residual oxides fromthe surface of the micromechanical device remaining following thetrenching process. Step 110 is followed by step 120, placing thesubstrate, wafer, and/or micromechanical device in an epitaxial reactor.Step 120 is followed by step 130, removing exposed sacrificial layermaterial by flowing H₂ gas at high temperatures within the device longenough to convert the sacrificial layer SiO₂ to gaseous H₂O and SiO andpermit these to be removed from the sacrificial layer and be borne outof the device. The removing operation may be performed at a pressurebetween about 1 millitorr and 100 torr, and may preferably be performedat a pressure of about 10 torr. Step 140 follows step 130 and marks theend of the micromechanical element release portion of a micromachineddevice manufacturing process.

In an alternative exemplary embodiment, Step 110 may not be performed,and the process proceeds from Step 100 directly to Step 120. In anotheralternative exemplary embodiment, an in situ cleaning step may beperformed between Step 120 and Step 130. The in situ cleaning step maybe performed in the epitaxial reactor and may include removing residualoxides from the surface of the micromechanical device by exposing thesurface of the device to H₂ gas and/or removing silicon residues byexposing the surface of the device to HCl. In a further exemplaryembodiment, a step of epitaxial deposition may be performed followingStep 130 before Step 140.

In another alternative exemplary embodiment, an SOI (Silicon onInsulator) wafer may be used in which a top silicon layer of the SOIwafer is the function layer and an insulator layer of the SOI wafer isthe sacrificial layer.

FIG. 4 shows handle wafer 41, which may be a silicon wafer, with devicelayer 42 arranged above handle wafer 41 and defining cavity 44 a. Cavity44 a may represent a space which was previously occupied by asacrificial layer. Encapsulation layer 43 is arranged on top of devicelayer 42 and includes vents 45 which access another cavity 44 b. In thismanner, device 46 may be released from handle wafer 41 below, and fromencapsulation layer 43 above, in one release step. In this manner, anexemplary device having multiple function layers and multiplesacrificial layers may be constructed. In an alternative exemplaryembodiment, more sacrificial layers and more function layers may bearranged above encapsulation layer 43.

In other alternative exemplary embodiments, there may be more than onesilicon layer to be released. For example, two vertically stackedsilicon layers with sacrificial oxide layers under and between them. Inthese more complex structures, the sacrificial oxide may be on top ofand beside, as well as on the bottom of, the structures.

While the present invention has been described in connection with theforegoing representative embodiment, it should be readily apparent tothose of ordinary skill in the art that the representative embodiment isexemplary in nature and is not to be construed as limiting the scope ofprotection for the invention as set forth in the appended claims.

1-21. (canceled)
 22. A device including micromechanical elements, comprising: a substrate layer; a sacrificial layer on at least a first portion of the substrate layer; and a function layer on at least a second portion of the sacrificial layer; wherein the function layer is released from the substrate layer by exposing the device to gaseous hydrogen.
 23. The device as recited in claim 22, wherein the device is produced in an epitaxy reactor environment.
 24. The device as recited in claim 22, wherein the function layer is released from the substrate layer by removing a third portion of the sacrificial layer, the third portion arranged between the function layer and the substrate layer.
 25. The device of claim 22, wherein: at least one of the substrate layer and the function layer include silicon; and the sacrificial layer includes an oxide bearing material.
 26. The device as recited in claim 25, wherein the substrate layer, the sacrificial layer, and the function layer collectively form an SOI wafer.
 27. The device as recited in claim 25, wherein: the sacrificial layer is deposited on the substrate layer; and the function layer is deposited epitaxially on the sacrificial layer.
 28. The device as recited in claim 22, further comprising: a further sacrificial layer on at least a third portion of the function layer; and a further encapsulation layer on at least a fourth portion of the further sacrificial layer; wherein the further sacrificial layer is released from the function layer by exposing the device to gaseous hydrogen. 29-40. (canceled) 